Dryer exhaust heat recovery

ABSTRACT

A system and method for extracting waste heat from the exhaust ( 20 ) of a biomass dryer ( 14 ), such as in a grain alcohol plant ( 10 ). A boiler circuit ( 74 ) provides steam at a high pressure to a balance of the plant ( 64 ). A recovered energy circuit ( 76 ) extracts heat from the exhaust and provides steam ( 60 ) at an intermediate pressure, thereby eliminating the need for a pressure reducing valve in order to satisfy an intermediate pressure steam demand in the plant. Working fluids in the boiler and recovered energy circuits are intermixed in a boiler feed vessel ( 72 ).

This application is a divisional of U.S. patent application Ser. No.15/496,413 filed 25 Apr. 2017, which in turn claims benefit of the 15Jul. 2016 filing date of U.S. provisional patent application No.62/362,785.

FIELD OF THE INVENTION

This invention relates generally to recovering heat from waste gasesproduced during industrial processing of materials, and morespecifically in one embodiment to recovering heat from the exhaust of adryer used to remove moisture from a wet biomass produced during theproduction of alcohol.

BACKGROUND OF THE INVENTION

The efficient utilization of energy is important in any industrialprocess, and it is well known to recover heat from a process gas priorto exhausting the gas back into the environment. The production of analcohol from corn or other biomass produces a moist solids byproduct(wet cake) which can be partially dried in a rotary drum or steam tubedryer before being further processed as animal feed or fertilizer. Thedryer exhaust gas contains heat and moisture that can be captured beforebeing released to the atmosphere.

U.S. Pat. No. 3,131,035 describes the recycling of dryer exhaust gasthrough an incinerator, with a portion of the incinerated gas beingexhausted to atmosphere only after passing through a heat exchanger topre-heat the bulk dryer exhaust gas travelling into the incinerator.That patent also teaches the extraction of heat from the exhaust gas viaa heat exchanger using a liquid extracted from the material being dried.The heated liquid is then concentrated in an evaporator which producesboth a concentrated liquid for reuse in the feedstock stream and a vaporthat is condensed and disposed of in any suitable manner.

United States Patent Application Publication No. US 2011/0108409 A1describes an ethanol production system where the exhaust stream from asteam dryer is directed to the bottom of a distillation column in orderto heat the distillation column and to scrub the exhaust stream.

U.S. Pat. No. 8,429,832 describes the use of waste heat from a steamdryer being captured and used in the production of steam for the dryer.

International Application Publication No. WO 2013/144438 A1 describesthe use of flue gas from a combustion device being used to heat a liquidto be concentrated in a multiple-effect evaporation plant.

Further improvements in the energy efficiency of such industrialprocesses are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following description in view of thesole FIGURE which is a schematic illustration of a grain alcohol plantembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have recognized that the benefit of capturingwaste heat from exhaust gas involves not only the way that heat istransferred from the exhaust gas into another fluid, but also the waythat the heated fluid is subsequently used to extract the recovered heatenergy. The inventors have also recognized that prior art energyrecovery systems which utilize fluids that are produced from or that arein contact with a biomass have limited flexibility due to the chemicalsincluded in such fluids. Moreover, simple recycling of a fluid to reuseheat energy is generally efficient, but it limits the use of therecycled energy to the heating of the fluid being recycled.

In contrast, the present invention provides both an efficient mechanismfor capturing waste heat from a biomass dryer exhaust gas and a highlyflexible mechanism for reusing the captured energy. This is accomplishedin one embodiment by utilizing a working fluid for the capture of thewaste heat, wherein the working fluid is compatible with a fluid used ina balance of plant process. In a grain alcohol plant embodiment of theinvention, one such working fluid is boiler feedwater quality water,which, after being heated by dryer exhaust gas, can be integrated into aboiler loop of a plant ethanol distillation process. Water is an idealfluid for heat transfer due to its heat capacity and its low cost. Thechemistry of boiler feedwater is typically closely controlled in orderto minimize corrosion and to maximize heat transfer in the boiler loop.For a particular plant, boiler feedwater chemistry requirements aretypically maintained in accordance with a specification specific forthat plant, and the term “boiler feedwater quality” as used hereinrefers to the chemical composition required by specification for aparticular plant's boiler loop.

The present inventors have also recognized an inherent inefficiency inthe boiler/steam systems of existing grain alcohol plants. Theinefficiency arises because those plants utilize steam at a number ofdifferent temperature/pressure conditions for a number of differentpurposes, including steam at a relatively high pressure for use inmolecular sieves, steam at intermediate pressures for use in feedstockheaters and thin silage evaporators, and steam at relatively lowpressure for use in the distillation columns. Because all of the plantsteam is provided by a single boiler operating at the highest requiredpressure, steam for lower pressure uses is typically provided via anenergy wasting pressure reduction device.

In contrast, an embodiment of the present invention minimizes such steampressure reduction losses by extracting heat energy from the dryerexhaust gas stream using boiler feedwater quality water, and convertingthat recovered energy into steam at an intermediate pressure for use inthe plant and subsequent return to the plant boiler circuit without theneed for a pressure reduction device. As will be described more fullybelow, this approach effectively bypasses the boiler circuit for some ofthe plant steam uses.

Reference is now made to the FIGURE which is a block diagram of anembodiment of the invention as applied in a grain alcohol plant such ascorn ethanol plant 10. A wet cake 12, such as corn or other biomasscontaining solids and water, is fed into an industrial dryer 14, such asa known rotary dryer or steam tube dryer. In the case of direct firedryers, a fuel 16 such as natural gas is used as the energy source fordrying the wet cake 12 to produce relatively drier solids 18. Dryerexhaust 20 evolved from the drying process may be directed through asaturation chamber or scrubber such as water scrubber 22 for twobeneficial reasons. First, much of the particulate matter is removedfrom the dryer exhaust 20 which greatly reduces fouling and buildup ofparticulate matter on the heat transfer area of the downstream dryerexhaust condensing economizer 24. Second, by bringing dryer exhaust 20into direct contact with a water source, the temperature of the dryerexhaust 20 drops and a proportional amount of water is evaporated. A dryscrubber may be used and will remove particulate matter, however, a wetscrubber, when optimized, will result in a fully saturated dryer exhaust26, thereby optimizing heat transfer in the downstream economizer 24.The scrubber 22 may be of any known design, such as a spray tower,cyclone spray chamber, venturi scrubber, orifice scrubber, impingementscrubber, packed bed scrubber, etc. The saturated dryer exhaust 26 exitsthe water scrubber 22 and is directed into the dryer exhaust condensingeconomizer 24 where it functions as a heating agent for a working liquidto be described more fully below. Water scrubber bottoms 28, water plusparticulate matter, is withdrawn from the water scrubber 22 at a rate asdictated by typical water scrubber design parameters. As the saturateddryer exhaust 26 passes through the dryer exhaust condensing economizer24, a portion of the water vapor within the saturated dryer exhaust 26will condense, producing dryer exhaust condensate 30. The dryer exhaustcondensate 30 and/or supplemental scrubber water 32 may be provided as awater source for the scrubber 22. The somewhat dehydrated dryer exhaust34 exits the dryer exhaust condensing economizer 24 and typicallyproceeds to a thermal oxidizer 36 before being passed to atmosphere 38.Much of the physical separation of the dehydrated dryer exhaust 34 andthe dryer exhaust condensate 30 may occur within the dryer exhaustcondensing economizer 24. To further aid in the separation of dryerexhaust condensate 30 from the somewhat dehydrated dryer exhaust 34, thesomewhat dehydrated dryer exhaust 34 may be routed through adisengagement step (not shown), such as a through a disengagement vesselhaving a mist eliminator, before being delivered to the thermal oxidizer36. Additional known heat removal equipment/processes (not illustrated)may be used to capture additional heat energy from the exhaust of thethermal oxidizer 36 prior to release of the gas to the atmosphere 38.

The working liquid used in the dryer exhaust condensing economizer 24 isa flash cooled liquid 40, which may be directed into the dryer exhaustcondensing economizer 24 for indirect thermal communication with thesaturated exhaust 26 in a counter flow direction or in a cross currentdirection as close to counter current as possible. The flash cooledliquid 40 picks up heat energy from the saturated dryer exhaust 26 andis withdrawn from the dryer exhaust condensing economizer 24 as heatedliquid 42. The heated liquid 42 is then directed to a flash vessel 44where the heated liquid 42 undergoes flash cooling, producing the flashcooled liquid 40 and flash vapor 46. The flash cooled liquid 40 beingrecirculated between the flash vessel 44 and the dryer exhaustcondensing economizer 24 forms a working fluid loop 48. A portion of thecirculating working liquid, preferably the flash cooled liquid 40, iswithdrawn as blowdown 50 in order to control the buildup of solids inthe system.

The flash vapor 46 is directed into the suction side 52 of athermocompressor 54. Plant steam 56 is directed into the motive side 58of the thermocompressor 54 in order to educe the flash vapor 46 into thesuction side 52 of the thermocompressor 54. The resulting steam mixture60 exits out of the discharge side 62 of the thermocompressor 54 at anintermediate pressure above that of the flash vapor 46 but below that ofthe plant steam 56. The steam mixture 60 can subsequently be used wherethere is demand in the balance of the plant 64 for steam at thecorresponding saturation temperature of the steam mixture 60. The term“thermocompressor” as used herein is generally meant to include othersimilarly functioning devices such as injectors, ejectors, jet pumps,exhauster, etc. which merge lower and higher pressure fluids to producean intermediate pressure fluid, such as by utilizing the venturi effect.

The plant steam 56 is provided by a boiler 66 which also provides plantsteam 56 for other high pressure steam demands in the balance of plant64, such as a molecular sieve (not shown). In this manner relatively lowquality flash vapor 46 is converted to a higher intermediate qualitymixture 60 in a very efficient manner, eliminating the need for reducingthe pressure of available plant steam 56 with a less efficient pressurereducing valve in order to satisfy an intermediate pressure steam demandin the plant 10. Spent steam is condensed and provided as condensate 70to a boiler feed vessel 72. Makeup liquid 68 is added to the flashvessel 44 in order to make up for the loss of mass from the fluid loop48 as flash vapor 46 and blowdown 50. Makeup liquid 68 may be introduceddirectly into the flash vessel 44, as illustrated, or at anotherlocation in the fluid loop 48. Makeup liquid 68 may be sourced fromcondensate 70 from the balance of plant 64, as illustrated, or fromother convenient sources such as from the boiler feed vessel 72 or froma boiler makeup water source (not illustrated). In some embodiments,blowdown 50 from the flash vessel 44 may be circulated directly orindirectly back to the boiler feed vessel 72.

It may be appreciated from the FIGURE that a boiler circuit 74 of theplant 10 includes the boiler feed vessel 72, boiler 66, and portions ofthe balance of plant 64 including a condenser (not illustrated). Arecovered energy circuit 76 exists in parallel to the boiler circuit 74and serves to provide steam for intermediate or low pressure uses in thebalance of plant 64. The recovered energy circuit 76 includes theworking fluid loop 48 moving heat from the dryer exhaust condensingeconomizer 24 into the flash vessel 44, as well as the thermocompressor54, portions of the balance of plant 64 and boiler feed vessel 72. Bothcircuits 74, 76 operate with intermixed and essentially identicalfluids, which in this embodiment of the invention is boiler feedwaterquality water.

Embodiments of this invention may utilize water such as steamcondensate, typically associated with a plant's steam system, as theworking fluid (the flash cooled liquid 40, heated liquid 42, and makeupliquid 68). One of the benefits of utilizing steam condensate, orsimilar water solutions, is the nearly identical composition of theflash vapor 46 to that of typical boiler derived plant steam. Utilizinga compatible liquid, such as plant steam condensate, yields identical ornearly identical condensate from the steam mixture 60 as compared totypical boiler derived plant steam condensate. This allows the presentinvention to be installed within the constructs of typical steam systemswithout the use of non-boiler compatible liquids. Other embodiments maybe envisioned where the working fluid is a non-water fluid that iscompatible with a non-water balance of plant system fluid, for example aclosed Rankine cycle using an alternative fluid such as ethanol ormethanol.

The present invention allows the recovered dryer exhaust heat energy tobe fully utilized in an efficient manner. Moreover, condensation ofwater from within the dryer exhaust gas facilitates the removal of somewater soluble pollutants that would otherwise either have to bedestroyed, typically in the thermal oxidizer 36, or emitted as pollutionto the atmosphere 38. The optional inclusion of the saturation step,i.e. water scrubber 22, is capable of removing even more potentialpollutants than the condensation alone. Energy consumed in the thermaloxidizer 36 can be estimated as the net increase in temperature betweenthe feed gases and the exit gases, multiplied by the specific heat ofthose gases, multiplied by the mass flow rate of those gases. Thecondensation of water from within the dryer exhaust by the presentinvention reduces the total mass flow of the dryer exhaust that entersthe thermal oxidizer 36, which subsequently reduces the amount of energyused and wasted during the downstream oxidization step.

Heat transfer efficiency in the dryer exhaust condensing economizer 24may be improved by increasing the pressure of the saturated exhaust 26to a value above the normal ambient pressure used to induce the flow ofthe gas through the system. This may be accomplished by including apressure increasing device such as a blower 78 in the exhaust flow pathanywhere upstream of the dryer exhaust condensing economizer 24 (e.g.upstream of the dryer 14, between dryer 14 and scrubber 22, or betweenscrubber 22 and economizer 24) and a pressure control device 80 in theexhaust flow path anywhere downstream of the dryer exhaust condensingeconomizer 24 and before venting to atmosphere 38. When upgrading anexisting plant to include this optional feature, excess capacity of anexisting blower of the dryer system may be utilized, subject to pressurelimitations in the remainder of the system components. A pressureincrease of only one inch of mercury may yield a twenty five percentimprovement in the log mean temperature difference in the dryer exhaustcondensing economizer 24.

In another embodiment, a plant including a dryer operable to removemoisture from a wet cake and to produce dryer exhaust gas comprisingwater vapor may include an improvement including: a dryer exhaustcondensing economizer having an exhaust gas inlet for receiving thedryer exhaust gas and an exhaust gas outlet for discharging dehydrateddryer exhaust gas, and further having a cooling fluid inlet and acooling fluid outlet of the dryer exhaust condensing economizer in fluidcommunication with a boiler circuit of the plant; and a saturationchamber disposed between the dryer and the dryer exhaust condensingeconomizer for saturating the dryer exhaust gas with water vaporupstream of the dryer exhaust condensing economizer exhaust gas inlet.The improvement may further include a conduit for delivering dryerexhaust condensate from the dryer exhaust condensing economizer to thesaturation chamber. The improvement may include a pressure increasingdevice disposed in a flow path of the dryer exhaust gas upstream of thedryer exhaust condensing economizer and a pressure control devicedisposed in the flow path of the dryer exhaust gas downstream of thedryer exhaust condensing economizer, the pressure increasing device andpressure control device cooperatively operable to control a pressure ofthe dryer exhaust gas in the dryer exhaust condensing economizer. Theimprovement may include: a flash vessel comprising a working fluidinlet, a working fluid outlet and a flash vapor outlet; a working fluidloop interconnecting the dryer exhaust condensing economizer coolingfluid outlet with the flash vessel working fluid inlet, andinterconnecting the flash vessel working fluid outlet with the dryerexhaust condensing economizer cooling fluid inlet; and athermocompressor having a suction side for receiving a flash vapor fromthe flash vapor outlet of the flash vessel and a motive side forreceiving plant process vapor, the thermocompressor operable to producea vapor mixture. Such a plant may be a corn ethanol plant and the plantprocess vapor and the vapor mixture may be steam.

While various embodiments of the present invention have been shown anddescribed herein, it will be obvious that such embodiments are providedby way of example only. Numerous variations, changes and substitutionsmay be made without departing from the invention herein. Accordingly, itis intended that the invention be limited only by the spirit and scopeof the appended claims.

The invention claimed is:
 1. A system for a plant, the plant comprisinga dryer operable to remove moisture from a wet cake and to produce dryerexhaust gas comprising water vapor, the plant also comprising a balanceof plant, the system comprising: a boiler circuit operable to providesteam at a first pressure to the balance of plant; a recovered energycircuit operable to extract heat from the dryer exhaust gas and toprovide steam at a second pressure, lower than the first pressure, tothe balance of the plant; and a component wherein liquid from the boilercircuit and the recovered energy circuit is intermixed or a componentwherein steam from the boiler circuit and the recovered energy circuitis intermixed.
 2. The system of claim 1, further comprising: therecovered energy circuit comprising a flash vessel in fluid loopcommunication with a dryer exhaust condensing economizer and operable toproduce a flash vapor; and a thermocompressor operable to receive theflash vapor and a portion of the steam at the first pressure and toproduce the steam at the second pressure.
 3. The system of claim 2,further comprising a makeup fluid flow sourced from the boiler circuitand delivered to the flash vessel fluid loop communication.
 4. Thesystem of claim 2, further comprising a blowdown flow from the flashvessel and directed to a boiler feed vessel of the boiler circuit. 5.The system of claim 2, further comprising a saturation chamber disposedbetween the dryer and the dryer exhaust condensing economizer forsaturating the dryer exhaust gas with water vapor upstream of the dryerexhaust condensing economizer.
 6. The system of claim 5, furthercomprising a conduit for delivering dryer exhaust condensate from thedryer exhaust condensing economizer to the saturation chamber.
 7. Thesystem of claim 2, further comprising a pressure increasing devicedisposed in a flow path of the dryer exhaust gas upstream of the dryerexhaust condensing economizer and a pressure control device disposed inthe flow path of the dryer exhaust gas downstream of the dryer exhaustcondensing economizer, the pressure increasing device and pressurecontrol device cooperatively operable to control a pressure of the dryerexhaust gas in the dryer exhaust condensing economizer.
 8. The system ofclaim 1, wherein the component wherein steam is intermixed comprises athermocompressor.
 9. The system of claim 1, wherein the componentwherein liquid is intermixed comprises a boiler feed vessel.
 10. A cornethanol plant comprising the system of claim
 1. 11. A grain alcoholplant comprising the system of claim
 1. 12. In a plant comprising adryer operable to remove moisture from a wet cake and to produce dryerexhaust gas comprising water vapor, the plant also comprising a boilercircuit and a balance of plant, a method of recovering heat from thedryer exhaust gas comprising: providing steam at a first pressure fromthe boiler circuit to portion of the balance of plant for a first steamuse in the balance of plant; extracting heat from the dryer exhaust gasvia a recovered energy circuit to provide steam at a second pressure,lower than the first pressure, to another portion of the balance ofplant for a second steam use in the balance of plant; and intermixing aliquid of the boiler circuit and a liquid of the recovered energycircuit in a boiler feed vessel.
 13. The method of claim 12, furthercomprising: producing a flash vapor in a flash vessel in fluid loopcommunication with a dryer exhaust condensing economizer of therecovered energy circuit; and combining the flash vapor and a portion ofthe steam at the first pressure in a thermocompressor to produce thesteam at the second pressure.
 14. The method of claim 13, furthercomprising sourcing a makeup fluid flow from the boiler circuit fordelivery to the flash vessel fluid loop communication.
 15. The method ofclaim 13, further comprising directing a blowdown flow from the flashvessel to a boiler feed vessel of the boiler circuit.
 16. The method ofclaim 13, further comprising saturating the dryer exhaust gas with watervapor in a saturation chamber disposed between the dryer and the dryerexhaust condensing economizer.
 17. The method of claim 16, furthercomprising delivering dryer exhaust condensate from the dryer exhaustcondensing economizer to the saturation chamber.
 18. The method of claim13, further comprising controlling a pressure of the dryer exhaust gasin the dryer exhaust condensing economizer by cooperatively operating apressure increasing device disposed in a flow path of the dryer exhaustgas upstream of the dryer exhaust condensing economizer and a pressurecontrol device disposed in the flow path of the dryer exhaust gasdownstream of the dryer exhaust condensing economizer.
 19. The method ofclaim 12, practiced in a corn ethanol plant.
 20. The method of claim 12,practiced in a grain alcohol plant.